1. Field of the Invention
The present invention relates to a mask, and more particularly, to a focus test mask for a projection exposure system, a focus monitoring system using the same and a focus monitoring method.
2. Description of the Related Art
In photolithographic processes for manufacturing a semiconductor device, a critical dimension and a profile of a pattern change greatly depending on the degree of focus in a projection exposure system. The shorter the exposure wavelength is, the more severe the change is. Thus, focus measuring and maintenance of the projection exposure system is very important in photolithographic processes.
A currently used projection exposure system includes an auto focusing apparatus, to realize optimal focus during exposure. However, as the exposing process proceeds, optimal focus cannot be achieved in an actual pattern even though the automatic focusing apparatus detects the optimal focus, resulting in inferior patterns. Thus, the automatic focusing system must be corrected to an appropriate value periodically, to allow the optimal focus to be sensed. The change in a reference for sensing the optimal focus of the automatic focusing apparatus may result from a step of a wafer surface which is an object, and irregularity of a thin film formed on the wafer. Also, change in the reference for sensing optimal focus may result when a pattern is formed by a projection optical system while automatic focusing is performed by a separate optical system, which causes different correction between two optical systems.
According to a general method for measuring the optimal focus of the projection optical system, a pattern of a specific type is formed while changing focus in an exposure system, and then changes in the critical size and profile of the pattern are checked. However, such a method is time-consuming and unreliable, i.e., subjective, since the conclusions depend upon visual inspection by an operator using a scanning electron microscope, and are subject to variation between operators or even between separate viewings by the same operator.
Thus, a new method for measuring focus using an automatic measuring apparatus has been proposed. This method is disclosed in U.S. Pat. No. 5,300,786, and is based on a shift of pattern depending on the degree of focus shift when a phase mask is used. That is, the degree of defocusing is calculated by measuring the shifted value of the pattern. However, the method uses a phase shift mask, so that it cannot be applied to a typical mask, e.g., a light shielding pattern mask such as a binary chrome mask.